Geographical Index Subject Index Bibliographical Reference British

BRITISH GEOLOGICAL SURVEY TECHNICAL REPORT WA/95/13 Onshore Geology Series

TECHNICAL REPORT WA/95/13 Geology of the Brixton Deverill-East Knoyle district (Wiltshire)

1:lO 000 sheets ST 83 NE (Brixton Deverill) and ST 83 SE (East Knoyle) Part of l:5O 000 Sheet 297 (Wincanton)

C R Bristow

Contributors B M Cox (Jurassic biostratigraphy) and M A Woods (Cretaceous biostratigraphy)

Geographical index UK, SW England, Wiltshire

Subject index Geology, Jurassic, Kimmeridge Clay Cretaceous, Upper Greensand, Chalk

Bibliographical reference Bristow, C R. 1995. Geology of the Brixton Deverill-East Knoyle district (Wiltshire). 1:lO 000 sheets ST 83 NE and ST 83 SE. British Geological Survey Technical Report WA/95/13

@ NERC Copyright 1995. Exeter, British Geological Survey 1995 PREFACE This account describes the geology depicted on 1:lO 000 sheets ST83NE (Brixton Deverill and ST83SE (East Knoyle), which form part of the l:5O 000 Wincanton (297) Sheet. The district was first geologically survyed on the one-inch scale by H W Bristow in 1852 as part of Old Series Sheet 15, published in 1856, and W T Aveline (date unknown) as part of Old Series Sheet 14, published in 1857. A revised edition of Sheet 14 was issued in 1859. Revision of the Cretaceous strata at the 1:lO 560 scale was carried out by A J Jukes-Browne in 1889 and 1890, and by F J Bennett in 1894-1896. Mr B H Mottram surveyed the area of the Mere Fault and associated disturbance at the 1:lO 560 scale in 1950 and 1951 and published the results in the Proceedings of the Geologists' Association in 1961; he later donated his maps to the BGS. A Provisional edition, at the 1:63 360 scale, of the Wincanton Sheet, based on Old Series sheets 14 and 15 and Mottram's mapping, was published in 1969. The sheet was reprinted at the l:5O 000 scale without geological revision in 1972. The 1:lO 000-scale revision survey of the district was carried out by C R Bristow in 1994 under the direction of R W Gallois and I R Basham, Regional Geologists. Cretaceous macrofossils have been identified by M A Woods. Dyeline black and white copies of the geological maps can be ordered from the British Geological Survey, Keyworth. CONTENTS Page 1. INTRODUCTION 1 2. JURASSIC Kimmeridge Clay Portland Group Wardour Formation Portland Stone Formation Tisbury Member 3. CRETACEOUS 9 Gault 9 Upper Greensand 10 Cann Sand 10 Shaftesbury Sandstone 10 Boyne Hollow Chert 11 Melbury Sandstone 11 Details 11 Chalk Group 14 Lower Chalk 14 West Melbury Marly Chalk 15 Zig Zag Chalk 15 Middle Chalk 15 Holywell Nodular Chalk 15 New Pit Chalk 16 Upper Chalk 16 Lewes Nodular Chalk 16 Seaford Chalk 17 Details 1.7 4. STRUCTURE 27 5. DRIFT Clay-with-flints Older Head Head Undifferentiated River Terrace Deposits First River Terrace Deposits Alluvium Lands lips Swallowholes 6. ECONOMIC DEPOSITS Building Stone Roads tone 31 Lime and Mar1 31 Brick Clay 31 7. REFERENCES 32 Appendix 1. Palaeontological reports relevant to the district 33 Figures 1. Geological sketch-map of the Brixton Devrill-East Knoyle 3 2. Sketch-map of the landslips and Kimmeridge Clay in the West Knoyle area 5 3. Sketch of hardgrounds in the Chalk Rock 22 4. Section in the Lewes Chalk at the East Knoyle Chalk pit [8875 31101 (based on Jukes-Browne and Hill (1904) and Woods (1994~)) 25 5. Sketch-map of the Mere Fault and associated monoclinal structure 28 Tables 1. Geological succession in the Brixton Deverill-East Knoyle area 2 1.INTRODUCTION The Brixton Deverill-East Knoyle district lies at the western endof Salisbury Plain and encompasses the north-western part of the Vale of Wardour. The central part of the district forms part of a dissected plateau developed on Upper Chalk (Figure 1); this reaches a maximum height of 238 m south-east of Brixton Deverill. In the north-west, there are prominent escarpments capped by the Lewes Chalk on either side of the Wylye valley. In the south, the Mere Fault and associated monoclinal structure play an important part in shaping the landscape. In the west, the chalk rises steeply on the north side of the fault from the clay vale to the south. Between West Knoyle and East Knoyle, the steeply dipping Upper Greensand and Chalk strata give rise to strongly featured ground. The principal drainage in the northern part of the district is northwards by the River Wylye, the only permanent river on the chalk outcrop and its tributaries. In the south-central area, drainage is eastwards by a series of valleys that coalesce just west of Hindon and ultimately join the River Nadder at Tisbury. In the south, on the clay vale, drainage is south- westwards by tributaries of the River Lodden, and south-eastwards by tributaries of the River Nadder. The lowest point in the district, c.96 m OD, lies in the southern tract. Soils developed on the Upper Greensand and most of the Chalk are light and well drained. However, soils on the West Melbury Chalk, together with some on the clay-with-flint deposits and Kimmeridge Clay are much heavier and poorly drained. Agriculture is a mixture of arable and pasture, with the latter dominant on the Kimmeridge Clay Vale. There are few woods, with deciduous woods confined mostly to the clay vale and the relatively newly planted coniferous plantations on the Chalk and Upper Greensand. Agriculture is the only industry in the area. The soild formations and drift deposits on the 1:lO 000 geological sheets ST83NE and SE are shown in Table 1.

1 Table 1. Geological succession in the Brixton Deverill-East Knoyle district

Drift deposits Landslip Alluvium River Terrace Deposits Undifferentiated River Terrace Deposits Head Older Head Clay-with-flints

So 1id Deposits Thickness (m) Chalk Seaford Chalk 45 Lewes Nodular Chalk 25-33 New Pit Chalk 12-15 Holywell Nodular Chalk 12-15 Zig Zag Chalk 10-25 West Melbury Chalk 15-40 Upper Greensand Melbury Sandstone 1-5 Boyne Hollow Chert 9-12 Shaftesbury Sandstone 1-5 Cann Sand 12 Gault 18 Portland Group Tisbury Member 10 Wardour Formation 15 Kimmeridge Clay 185

2 e

I I. I I I Seaford Chalk LeCk Lewes Nodular Chalk

Middle Chalk

Zig Zag Chalk West Melbury Marly Chalk Upper Greensand

Gault

Kimmeridge Clay

/-Fault, crossmark on downthrow side

Figure 1, Sketch-map of the solid geology of the Brixton D everill - East Knoyle district 2. JURASSIC Kimmeridge Clay About 185 m of Kimmeridge Clay crop out in the southern part of the district, of which about 30 m belong to the upper part of the Lower Kimmeridge Clay and the rest to the Upper Kimmeridge Clay. The formation consists dominantly of medium grey, fissile, shelly mudstones, and medium and brownish grey, bituminous shelly mudstones ('oil- shale'). Cementstone beds occur at intervals and appear to form mappable features at three principal levels (Figure 2). The lowest, which is exposed in the stream bed [8653 30761 north-east of Lugmarsh Farm, is a lobate, dense, ?ferroan, highly septarian cementstone which appears to fall in the basal part of KC35 or in the top part of KC34 (at about the level of the Maple Ledge Stone Band of the Dorset coast). The second cementstone, which occurs about 5 m higher, is exposed in the stream bed [8608 31221 north of Lugmarsh Farm, It is a 15 cm thick, brownish grey, densely cemented, septarian cementstone with a fluted top. It is overlain by a fissile, shelly, oil shale. The exact position of this cementstone (Figure 2, locality 8) within the Kimmeridge Clay is problematical as the associated fauna (see details) indicates KC37 near the base of the Upper Kimmeridge Clay (and therefore Yellow Ledge Stone Band), which conflicts with the evidence of nearby faunas downstream [8599 31151 (Figure 2, locality 7) and upstream [8610 3131 and 8613 31371 (Figure 2, localities 9 and 10). Locality 10 yielded the ammonite AuZacostephanUs, and is therefore firmly fixed in the Lower Kimmeridge Clay (KC35 or below). A presumed cementstone, some 13 m higher than the second, forms an excelIent feature across the present area and on to the adjacent district to the south where it was thought to represent the Yellow Ledge Stone Band at the base of KC37 (Bristow, 1993). However, in the present area, fauna1 evidence suggests that feature represents a stratigraphically lower level than the Yellow Ledge Stone Band, and possibly lies at the Lower/Upper Kimmeridge Clay boundary (i.e. Blake's Bed 42 of the Dorset coast). The oldest strata, belonging to the Eudoxus Zone (KC3O), crop out in the unnamed stream on the extreme west of the district, south-west of West Knoyle [8504 31821 (Figure 2, localities 1-2). Younger strata, belonging to the Autissiodorensis Zone, have been recognised in streams south-west, south, and south-east of West Knoyle.

4 -31

West Knoyle area Although some 155 m of Upper Kimmeridge Clay crop out in the district, there is no undoubted exposure in this unit. There are possible occurrences of the basal bed (KC36) of the Upper Kimmeridge Clay in two streams [8610 3131 and 8657 30851, but the fauna1 evidence is equivocal (see Details) and it is thought more likely that these two localities fall in the top part of KC35. One possible occurrence of KC37 [8608 31221 has been mentioned above.

Details In the stream west of Broadmead Farm, West Knoyle, moderately fissile, medium to pale grey, moderately shelly and shelly mudstones with Nanogyra virguta, Protocardia and rare Amoeboceras (Nannocardioceras) krausei crop out [8504 3182](Figure 2, locality 1). An associated cementstone yielded Nanogyra virguta and a large Liostrea. An exposure [8505 31841 further upstream (Figure 2, locality 2) of moderately fissile, medium grey, moderately shelly and shelly mudstones includes abundant Protocardia morinica, Nanogyra virguZa and rare Amoeboceras (Nannocardioceras) anglicum. The fauna from these two localities indicates the Eudoxus Zone (?KC3O), A third exposure [8515 31861 (Figure 2, locality 3) still further upstream is in pale and medium grey, rather smooth-textured mudstones with several Nanogyra uirguZa and an indeterminate AuZacostephanus fragment; the fauna indicates the Lower Kimmeridge Clay. Farther south, another stream section [8501 31171 (Figure 2, lacality 4) exposes fissile, medium and brownish grey, tough, shelly mudstone with abundant AuZacostephanus including A. ex gr. vozgensis and A. faZZaz, indicative of the Autissiodorensis Zone, KC34 or, less likely, KC35. A second exposure [8500 31161 close by (Figure 2, locality 5) of a similar lithology close by yielded Protocardia morinica and AuZacostephanus including A. ex gr. uo Zgens i s. Sections along a third stream farther east are all in the Autissiodorensis Zone. The lowest exposure [ 8579 30371 (Figure 2, locality 6) is in fissile, medium grey, bituminous, moderately shelly mudstones with Isocyprina minuscuta, Nanogyra virguZa, a worn impression of a large ammonite ( ?Aulacostephanus aut issiodorensis) and several Amoeboceras (Nannocardioceras) probably indicative of KC34. Farther upstream [8599 31151 (Figure 2, locality 7), moderately to sparsely shelly mudstones with bivalves including Corbulomima suprajurensis, Nanogyra uirguZa and Protocardia morinica and ammonite fragments including a possible Subdiehotomoceras websteri suggestive

6 of KC35 or, less likely, KC34. Some 70 m farther upstream, a densely cemented, brownish grey cementstone with a fluted top forms a small waterfall (Figure 2, locality 8). This is the middle of the three feature-forming beds of the district. From fissile oil shale, estimated to be about 0.3 m above the cementstone were obtained Nanogyra virguta, clusters of NicanieZZa eztensa, abundant Protocardia, clusters of small trochiform gastropods (SemisoZarium haZZami) and pectinatitid ammonite fragments apparently suggestive of the Scitulus Zone, KC37. However, as mentioned above, this assignment is at variance with the ages of nearby strata both downstream and upstream (see below). For the moment, the problem remains unresolved. Continuing upstream [8610 31311, fissile, brownish grey, moderately shelly mudstone with ?Nanogyra virguta, Protocardia morinica and pectinatitid fragments, about 5 m below the highest feature-forming (presumed cementstone) bed (Figure 2, locality g), probably fall in KC35 or, less likely, KC36. The highest locality [8613 31371 collected (Figure 2, locality 10) consisted of fissile, brownish grey mudstones with Aulacostephanus fragments and Corbulomima suprajurensis indicative of KC35 or, less likely, KC34. A fourth stream section [8657 30851 (Figure 2, locality 11). the most easterly examined, revealed fissile, medium and brownish grey, shelly mudstones with CorbuZomima suprajurensis, Nanogyra virguZa, Protocardia morinica, the limpet-type gastropod PseudorhytidopiZus Zatissima and ?pectinatitid ammonite fragments suggestive of the Autissiodorensis Zone, KC35, or, less likely, Upper Kimmeridge Clay, Elegans Zone, ~~36.These mudstones are associated with a very distinctive, highly reticulate, ?ferroan cementstone, which lies about 5 m lower than the middle feature-forming bed which forms the small waterfall in the third stream section above.

7 Portland Group Strata of the Portland Group crop out only in the south-east of the district. The group is divided into two formations, a lower, Wardour Formation, and a higher, Portland Stone Formation, of which only the lower part of the latter crops out in the district.

Wardour Formation Formerly known as the Portland Sand, the term Wardour Formation is a slight modification of the Wardour Member of Wimbledon (1976). The formation consists of clayey sands and sandy clays; the base is commonly marked by springs. About 15 m appears crop out in the district, but from evidence on the adjacent sheets to the east and south-east, the real thickness is usually much more, with the thickness seen at outcrop usually much reduced by cambering of the overlying limestones.

Portland Stone Formation Where fully developed in ground to the east, the Portland Stone Formation is divisible into three members, but only the lower, the Tisbury Member, crops out in the present district.

T i sbury Member The Tisbury Member was formerly known as the Lower Building Stones (Blake and Hudleston, 1877 etc; Woodward, 1895). The latter were subdivided by Wimbledon (1976) into a lower, Chicksgrove Member, and an upper, Tisbury Member. In the field, it has not been possible to map separately the Chicksgrove Member and so the term Tisbury Member as used in this account has been extended downwards to include the Chicksgrove Member. The Tisbury Member consists of glauconitic, bioclastic sandstones and limestones. Some 10 m crop out in the district. In adjacent areas, the stones have been extensively worked for building stones.

8 3. CRETACEOUS Gault On the old One-inch map of 1856, the Gault is shown as dying out east of East Knoyle. However, Jukes-Browne who resurveyed the ground at the six-inch-to- the-mile scale in 1890, correctly identified the outcrop of the Gault, and noted that its surface was much 'obscured by rainwash from the Greensand above' (Ms, BGS). The Gault consists of up to 20 m of glauconitic, micaceous, fine-grained sandy clay. Over much of the district, the beds are steeply dipping, with a consequence that there is only a narrow outcrop. However, around Windmill Hill [870 3101 the dip flattens and the outcrop widens. No diagnostic fauna has been found in the area; the only recorded fossil is 'Ammonites [Hoplites] splendens' from a well on the west side of East Knoyle (Jukes-Browne Ms, BGS).

Details Jukes-Browne (Ms, BGS) noted that at West Knoyle, 'in the narrow part of the field south-east of Manor Farm there is a boggy place [c. 8611 32591 where weak springs break out, and in this I found a soft ferruginous sandstone or rotten ironstone containing large quartz grains and pebbles as big as a pea. This I took to be the base of the Gault'. An auger hole [8688 31771 at Upton proved 1 m of clayey, fine-grained sand, on 0.2 m of medium grey, fine-grained, micaceous sand. A well [8758 30473 was sunk in a field west of the Rectory in 1890 and was, at the time of Jukes-Browne's visit, about 20m deep, the section being as follows: Thickness m Soil 0.6 Gault Brown and yellow clay 2.4 Dark grey silty clay 6.1 Dark green glauconitic sand about 6.1 Kimmeridge Clay Dark grey clay 6.1

On the spoil heap from the well he found several small phosphatic nodules and fragments of ammonites, one being apparently 'Ammonites spZendens'. The 6 m of glauconitic sand at the base of the Gault seems excessive, as augering elsewhere in the deposit has not proved such a thickness.

9 At the cottage south of this field, and at a level 5 or 6 m lower, there is another well [8759 30401, about 10 m deep. After passing through 'clay and silt, a harder rocky bed was encountered, but not too hard to be worked with a pick; water was found below this in sufficient quantity and pretty good, but in the hope of finding another spring, they bored 4.5 or 6 m deeper through clay without success'. Jukes-Browne thought that the rocky bed was the base of the Gault, resting on Kimmeridge Clay, but it is more probable that it is a cementstone bed within the Kimmeridge Clay. At East Knoyle, a combination of temporary section and augering [8813 30391 proved 1.8 m of stony, sandy, glauconitic clay, on 0.3 m of dark grey, sandy clay.

Upper Greensand In adjacent areas, the Upper Greensand is divisible into a four-fold ascending sequence of Cann Sand, Shaftesbury Sandstone, Boyne Hollow Chert and Melbury Sandstone. Because of the steep dip (up to 45') along much of the outcrop, the width of each member is narrow, except on dipslopes. Because the Melbury Sandstone is very thin (maximum of 2 m thick), combined with the steep dip, it means that it has not been possible to map this unit separately.

Cann Sand The Cann Sand consists of about 13 m of glauconitic, fine-grained, micaceous sand and weakly cemented sandstones. It is equivalent to the 'Malmstone' of Jukes-Browne and Hill (1900). The base is commonly marked by springs.

Shaftesbupy Sandstone The Shaftesbury Sandstone consists of about 15 m of glauconitic, fine-grained and weakly cemented sandstone, At the top, there is a calcareously cemented, bioclastic sandstone known as the Ragstone. The base is usually marked by pronounced negative feature break and the member forms a scarp face rising abruptly from the shelf formed by the Cann Sand. The Ragstone was commonly worked as a building stone, and there were several quarries formerly opened in the district to exploit this stone. Except in the Ragstone, fossils generally are scarce, but include Amphidonte obliquatum, 'Pectens' and Pycnodonte (?Phygraea) vesiculosum.

10 Boyne UoZZow Chert The Boyne Hollow Chert consists of between 9 and 12 m of glauconitic, fine- grained sand with common interbeds of chert nodules. The chert nodules form a common component of the brash developed on this member. The chert beds were commonly worked for roadstone. No fossil has been found in the Boyne Hollow Chert of the district.

MeZbury Sandstone The Melbury Sandstone consists of between 1 and 2 m of fine-grained, weakly glauconitic, fossiliferous, calcareous sandstone. Sections have been seen in the past at two localities, but there has been no recent exposure. Fossils, especially, brachiopods, bivalves, ammonites and echinoids usually abound. However, much of the fauna is phosphatised and abraded and is probably reworked. Evidence from nearby areas (Bristow, 1994) indicates that carcitanense Subzone fossils are probably derived, and the Melbury Sandstone is more likely to belong to the saxbii Subzone of the manteZZi Zone and/or the higher dixoni Zone.

Details Shaftesbury Sandstone Old quarries [8622 32641 at the western end of Cleeve Hill, West Knoyle, have small exposures of friable, glauconitic sandstone with Amphidonte oblipatum. There are several pits at the eastern end of Cleeve Hill. The most westerly [8691 32381 formerly exposed the junction of the Boyne Hollow Chert and Shaftesbury Sandstone, with the strata dipping at 42' north-eastwards. At the present day, a section on the south side of the pit shows 0.4 m of fine- grained, friable, glauconitic sandstone, overlain by 1.5 m of harder glauconitic sandstone; chert beds are exposed on the north side of the pit. Farther south-east along the hill, another pit [8701 32311 exposes friable , glauconitic sandstone dipping north-eas twards at 35 ' . A third pit [8707 32271, also in friable, glauconitic sandstone, yields common Pycnodonte (Phygraea?) vesiculosum. It appears to be this pit in which Jukes-Browne (Ms, BGS) noted 2.7 or 3 m of sandstone 'of the usual character' with common pectens, including 'P. asper, P. orbicularis, P. interstriatus, P. quinquecostatus and SpondyZus striatus ' . Close by, and presumably lying south of the above pit was a sandpit, unfortunately not located, where Jukes-Browne (Ms, BGS) saw 3.6 m of laminated green sand with 'Pecten

11 quadricostatus and Exogyra conica' in one layer of it. The lamination is made visible by the varying amount of glauconite, some layers containing so much as to appear dark green, while in others, quartz is the principal ingredient. This passes down into buff sand with ferruginous stains and finally into firm, buff, micaceous sand, the thickness of these sands being perhaps 6 m, but as there is a northerly dip of 29' here it is not easy to estimate thicknesses. Between Upton and Milton, the Shaftesbury Sandstone forms well-featured ground. On Haddon Hill, where the dip is quite steep to the north-east, it has a narrow outcrop, but near The Green and west of Clouds House, where the beds are horizontal or with only a slight east-north-easterly dip, the outcrop is wider. In the latter area, the Shaftesbury Sandstone is about 20 m thick. The large sand pit [8760 30631 below Knoyle Hill showed the following section (Jukes-Browne, Ms, BGS): Thickness m Sandy soil 0.30 Boyne Ho 1 low Chert Pale grey sand with whitish sponge rock and some nodules of chert 1.22 Shaftesbury Sandstone Soft, greenish grey sand 0.46 Soft, greenish grey, sand-rock, consisting of coarse quartz and glauconite grains (not consolidated as it is elsewhere) 2.90 Soft, green sands, with a few hard siliceous concretions in the upper part, weathering into laminae -----10 97 15.85

The sand-rock contains 'Exogyra conica, Pecten interstriatus and large Pecten quadricostatus' . In the lower part of the pit are soft and yellowish micaceous sands, and in the lane below is micaceous sandstone [8756 30581 passing down into malmstone (Cann Sand) [8757 30551. The beds dip at about 10" north-north- eastwards. A sunken lane [8882 30821 north-east of East Knoyle shows glauconitic, fine-grained sandstone dipping northwards at 20" . Farther east along Knoyle Ridge, another section [8968 30691 (see Melbury Sandstone details) showed about 3.66 m of firm, glauconitic sand-rock, on soft, yellowish sands, micaceous below and passing down into micaceous sandstone (no thickness given) (Jukes-Browne and Hill, 1900 and Ms, BGS). The dip is very steep, about 30" to the north, and it is impossible to estimate the thickness of the sands below the sand-rock.

12 Boyne Hollow Chert Chert is common as brash on the crest of Cleeve Hill, West Knoyle. Chert beds are exposed on the north side of a pit [8691 32381 on Cleeve Hill, West Knoyle, where they were described (Jukes-Browne and Hill, 1900) as consisting of soft, marly sand with many thick, irregular layers of whitish sponge rock, and some lenticular masses of black and brown chert near the top. Between Upton and Milton, the outcrop of the Boyne Hollow Chert broadens and brash of chert is common on the surface. Chert was formerly dug, presumably for road metal, from small pits [8772 3160 and 8770 31781, but there is now no exposure. Mottram (Ms, BGS) noted the junction of the 'Warminster Greensand' and the chert beds in a lane [8798 31251 at Milton. The Boyne Hollow Chert was formerly exposed at the top of pits [8760 30631 on Knoyle Hill and south of Knoyle Corner [8968 30691 (see above).

Melbury Sandstone A pit [8725 32251 at Upton formerly exposed a section from the Boyne Hollow Chert to the West Melbury Marly Chalk (Jukes-Browne, Ms BGS; Mottram, Ms, BGS) . There, Jukes-Browne (Ms, BGS) noted grey sandy chalk on the eastern side of the pit, and fragments of hard, glauconitic mar1 on the south side. The dip is 35O north-east. This is presumably the pit at 'Upton' from which J Rhodes collected an extensive fauna. However, the locality details on the BGS register state that the pit lies north-west of Upton. This is believed to be a typographical mistake and should read north-east, since there is no pit to the north-west of Upton in which the West Melbury Marly Chalk was exposed. The fauna from the 'Upper Greensand' (Melbury Sandstone) includes worn and phosphatised specimens of Boubeithyris diploplicata, Dereta pectita, Grasirhynchia grasiana, ?Ovatathyris ovata, Aucellina grgphaeoides, Entolium orbiculare, Grgphaeostrea canabiculata, Lima subovalis, Mimachlamys ex gr. robinaldina, Rastellum (Arctostrea) ex gr. carinatum, Schloenbachia varians, Discoides subuculus and Salenia (S.) petalifera, indicative on face value of the mantelli Zone, carcitanense Subzone, but the fauna may well be reworked into a younger subzone or zone. From the 'Chloritic Marl' in the pit were obtained Cucullaea aff. mailleana, Plicatula inflata, Mantelliceras aff. couloni and M. mantelli (phosphatised), which, if indigenous, could indicate the mantelZi Zone, carcitanense Subzone, but the fauna may well be reworked. Mantelliceras couloni characterises a Lower Cenomanian horizon at the top of the carcitanense Subzone ('couloni Horizon'), in the mantelli Zone ( WrightKennedy , 1984) . In the lane south of Knoyle Corner [8967 30701, Jukes-Browne and Hill (1903) recorded the following section:

Thickness m Chalk Mar1 0.91 Melbury Sandstone Glauconitic mar1 9. Hard, sandy and glauconitic chalk 0.15 8. Soft, glauconitic chalk, marly at top, very sandy below 0.76 Rye Hill Sands 7. Very hard, glauconitic sandstone with a few phosphates (see P. 1 0.61 6. Line of small decomposed ferruginous nodules 0.05 5. Hard, coarse, glauconitic sandstone with large quartz grains (contains Pecten asper) 0.30 4. Soft yellowish sand 0.91 Boyne Hollow Chert 3. Greyish sand with layers of chert 9.14 Shaftesbury Sandstone 2.Firm, glauconitic sand-rock about 3.66 1. Soft, yellowish sands, micaceous below and passing down into micaceous sandstone ?

The dip is about 30" to the north. There is a complete passage from the upper sands to the Chalk, and the interbedding of hard and soft glauconitic marls is noticeable.

Chatk The Chalk is divided into three formations, the Lower, Middle and Upper Chalk, which in turn are divided into members, each easily recognisable by its lithology. In the present district, only the two lowest members of the Upper Chalk are preserved.

Lower Chalk The Lower Chalk consists of a lower unit, the West Melbury Marly Chalk, between 15 to 40 m thick, of buff, cream and off white marly chalk with thin interbeds of hard chalk, and an upper unit, the Zig Zag Chalk, between 18 and 25 m thick, dominantly of white and off-white, blocky chalk.

14 West Melbury Marly Chalk The member has a narrow outcrop in the zone of steep dips in the south of the district. In the north, in an area of gentle dip, the West Melbury Marly Chalk forms a prominent shelf, at least 500 m wide, at the foot of the escarpment formed by the Zig Zag Chalk. The dominantly marly nature of the West Melbury Marly Chalk gives rise to heavy clay soils. The unit appears to be thinner in the south, about 15 m thick, and thicker, up to 40 m thick, in the north of the district. Fossils are locally common, especially the ammonite Schloenbachia varians, but none have been recorded in the present district. Evidence from nearby (Bristow, 1994) indicates that the member ranges from the dixoni Zone to the rhotomagense Zone, costatus Subzone.

Zig Zag Chalk The Zig Zag Chalk forms the base of the main chalk escarpment and rises steeply from the shelf formed by the West Melbury Marly Chalk. The member consists dominantly of blocky, off-white and white chalk; at the top, but not exposed in the district, is an alternating sequence of greenish grey marls and off-white chalk - the Plenus Marls. Fossils are not common, but included inoceramids, oysters and ammonites (particular Acanthoceras).

Middle Chalk The Middle Chalk comprises two units of more or less equal thickness, a lower, Holywell Nodular Chalk and a higher, New Pit Chalk. The thickness of the Middle Chalk is constant across the whole district at about 25 m. Ad[@?3 36521 at Lower Pertwood Farm proved 24.2 m of Middle Chalk (Jukes-Browne and Hill, 1903).

Holywell Nodular Chalk The member consists dominantly of hard nodular chalk. There are marly wisps between some of the nodules. The base of the Holywell Nodular Chalk is sharp, with hard, commonly ironstained, nodular chalk resting abruptly on marly chalk or mar1 of the Plenus Marls. In the field, the hard basal beds commonly form a positive feature, and the basal boundary of the member is one of the easiest of the chalk boundaries to map. In the higher part of the succession, the nodularity decreases and thin non-nodular beds are interbedded with nodular beds over an estimated 1 to 2 m transition zone with t~eoverlying New Pit Chalk. Specimens of Mytiloides, especially M. mytiloides and M. labiatus, both whole and fragmentary, are common in the middle and upper part of the Holywell Nodular Chalk.

New Pit Chalk The New Pit Chalk consists of firm, white, slabby chalk. Thin-shelled Mutiloides species (M. hercynicus and M. subhercynicus) of distinctly flattened morphology and typically lacking preserved shell, occur locally. As mentioned above, the basal boundary is transitional over a metre or so.

Upper Chalk Where fully developed, the Upper Chalk consists of five mappable members, but only the two lowest, the Lewes Nodular and Seaford chalks, are preserved in the present district. The base of the Upper Chalk is defined at the base of the first significant incoming of hard nodular chalk above the New Pit Chalk.

Lewes Nodular Chalk The base of the Lewes Nodular Chalk is well defined by the development of hard nodular chalk above the softer chalk of the New Pit Chalk. About 2 m above the incoming of nodular chalk, is a hard, massive, porcellanous chalkstone with four or five, glauconitised hardground surfaces, some 1 to 1.5 m thick; this unit is known as the Chalk Rock. Flints make the first significant appearance in the Chalk Rock. Collectively, these basal strata form a positive feature which, combined with the distinctive brash derived from the Chalk Rock, makes this, like the Holywell Nodular Chalk, one of the easiest chalk boundaries to map. The Chalk Rock is succeeded by hard, nodular, flinty chalk which gradually decreases upwards in hardness and nodularity and passes over a transition zone of some ?3 to ?4 m into the Seaford Chalk. The thickness of the member varies between 25 and 35 m. Fossils are locally common in the Lewes Nodular Chalk and consist of brachiopods, inoceramids and echinoids, especially Micraster species. The member ranges from the top part of the lata Zone, through the plana Zone to the top of the cortestudinarium Zone. In the Shaftesbury area to the south, it locally extends into the early coranguinum Zone (Bristow et al., 1994).

16 Seaford Chalk The Seaford Chalk consists of up to 45 m of firm, white, flinty chalk. As noted above, the basal boundary is transitional over a few metres. Fossils, especially inoceramids, are common; the lower part of the member is typified by specimens of the inequivalved genus Volviceramus, whilst the sheet-like genus Platycerams is more long ranging, overlapping in range with Volviceramus at the base and extending throughout much of the rest of the member. Wicraster coranguinum can be found quite commonly. Within the present district, the lowest part (from evidence to the south, about 16 to 18 m of strata) of the Seaford Chalk falls in the late cortestudinarium Zone, butthe bulk of the Seaford Chalk falls in the eoranguinum Zone. The top is not preserved in the present district and the position of the highest strata within the coranguinum Zone is uncertain, but probably does not extend beyond the Coniacian part of the zone.

Details West MeZbury MarZy Chalk In the north of the district, the West Melbury Marly Chalk crops out on either side of the River Wylye from Brixton Deverill northwards to the edge of the district. There is no exposure, but the upper boundary with the Zig Zag Chalk is well defined by feature. In the north-west of the district, there is a wide, gently sloping, outcrop which gives rise to heavy clay soils. A borehole [8570 38051 which commenced almost at the top of the member proved 41.5 m of West Melbury Marly Chalk above the Upper Greensand. A second borehole [8590 38341 400 m north-east, sited on the floodplain of the River Wylye, which may have started in the base of the Zig Zag Chalk, proved 41 m of chalk above the Upper Greensand. In the south of the district, because of the steep dip, the member has only a narrow outcrop. This fact, combined with a persistent wash of sandy chert debris and lack of feature at the base of the Zig Zag Chalk, makes it difficult to separate the two members by augering alone. Augering consistently proves greyish brown clay or marl. There are few exposures. A small scrape [8649 32651 at West Knoyle in off-white and pale grey marly chalk yielded Inoceramus sp. and Limaria elongata (Woods, 1994~). The basal beds must formerly have been exposed in the pit [8725 32251 at Upton where 'Chalk Marl' was recorded (Jukes-Browne, Ms BGS; Mottram, Ms, BGS). The 'Chloritic Marl' noted by Jukes-Browne between the Boyne Hollow Chert and the Chalk Marl in this pit is regarded herein as part of the Melbury Sandstone (see details). Grey marl can be augered in the sides of the pit. The basal beds were also exposed at Knoyle Corner [8967 30701, where Jukes-Browne and Hill (1903) noted 0.9 m of 'Chalk Marl' above the Melbury Sands tone.

Zig Zag Chalk In the north-west of the district, there are small exposures C851.8 3911, 8525 3928, 8529 3934, 8547 3952 and 8594 39583 on the north and north-west side of Brims Down. Marly chalk and marl close to the top of the member and exposed in the banks of the track [8579 39131 on the south-east side of Brims Down is probably part of the Plenus Marls. Near Monkton Deverill, firm, white, blocky chalk is exposed in scrapes 18553 3793 and 8557 37971 on the steep bank of the old river cliff. North-east of Brixton Deverill, marly chalk in a ploughed field [8729 39753 is probably part of the Plenus Marls. A little farther east, blocky, white chalk [878O 3967 and 8788 39701 was ploughed up a short distance below nodular chalk of the Holywell Nodular Chalk. In the south of the district, because of the steep dip, there is only a narrow outcrop of the Zig Zag Chalk and that is distinguished with difficulty from the West Melbury Marly Chalk. At West Knoyle, on the steep, south-facing bank of The Warren, greyish green marl of the Plenus Marls was exposed beneath nodular chalk of the Holywell Nodular Chalk in a small scrape [8664 32681. Blocky white chalk close to the top of the member was seen beneath head deposits in a cutting behind a barn [8706 32411 at Chapel Farm, Upton. The large pit [8971 30781 at Bull Pits east of East Knoyle must formerly have exposed a good section in the member. The pit is now overgrown, but there are scrapes in the side of massive, blocky, white chalk. Jukes-Browne (Ms, BGS) recorded 6 m of greyish white chalk 'probably nearer the top than the base, as the Melbourn Rock crops out in the bank above the pit.

Holywell Nodular Chalk In the north-west of the district, the member is about 15 m thick. Nodular chalk, commonly with Plytiloides is common on the hillside all the way around Fir Clump and southwards to Whitecliff Farm and beyond. In the banks [8525 39081 on the south side of Fir Clump, nodular chalk close to the base of the

18 member yielded Orbirhynchia?, Myti Zoides myti Zoides and M. Zabiatus (Woods, 1994b) . Near Monkton Deverill , nodular chalk includes common Myti Zoides mytizoides [8528 3765, 8516 3759 and 8549 38021 and Orbirhynchia aff. herbeti [8543 36981 (Woods, 1994b) . South of Monkton Deverill, nodular chalk with rhynchonellids [8543 36991 was noted at one point. An old quarry [8596 37051 just off the Hindon Road is overgrown, but nodular chalk with MytiZoides is visible in small scrapes. On the eastern side of the Wylye valley, nodular chalk with FlytiZoides can be found all along the valley sides. North-east of Brixton Deverill church, Myti Zoides myti Zoides [8674 39061, Inoceramus ex gr. pictus [8672 39091 and Mytitoides Zabiatus and M. mytitoides E8695 39481 were found in brash. The specimen of I. pictus was found in very hard chalk at the base of the Holywell Nodular Chalk, and was overlain by nodular chalk with common Myt i Zoides . The outcrop of the Holywell Nodular Chalk can be readily traced by the nodular chalk brash with MytiZoides along the valley sides around Lord's Hill Farm and Sutton Bottom. In this latter area, where the Holywell Nodular Chalk is about 20 m thick, MytiZoides mytizoides is common [8844 39861 (Woods,

1993d) * A well [8830 36521 at Lower Pertwood Farm proved 24.2 m of Middle Chalk (Jukes-Browne and Hill, 1903). The Melbourn Rock at the base is described as 2.6 m of 'hard chalk, partly very hard'. In the south of the district, because of the steep dips associated with the Mere structure, the outcrop of the member is very narrow and there are few exposures. The junction with the Plenus Marls [8664 32681 was noted on the north side of The Warren. Close by, and 2 or 3 m higher, nodular chalk with Inoceramus sp. was exposed in a small scrape [8656 32701. Hard nodular chalk with MytiZoides mytizoides crops out on the hill top [8666 32641 by the triangulation point east of West Knoyle (Woods, 1994b). At Chapel Farm, Upton, nodular chalk was exposed beneath head deposits in excavations [8704 3241 and 8707 32441 behind a barn. At East Knoyle, an old pit [8875 31091 formerly exposed the whole thickness of the Holywell Nodular Chalk. Jukes-Browne and Hill (1903 and Ms) noted that rough nodular chalk of the Melbourn Rock was exposed behind the lime-kiln, and that at the base of the quarry, there was a 0.6 m thick bed of hard, cream, nodular chalk overlain by a greenish marl, and then a vertical face of New Pit Chalk (4.v.). Hard, nodular chalk was exposed at the top of the Bull Pits [8973 30821 on the eastern margin of the district.

New Pit Chalk A narrow belt of firm, white chalk can be traced all the way round the hillside at Fir Clump in the north-west of the district. South of Fir Clump, firm, white, slabby chalk with Mytiloides ex gr. hercynicus-subhercynicus occurs as brash [8560 3878 and 8570 38631 (Woods, 1994b). From Monkton Deverill southwards, brash of firm white chalk was only noted intermittently. On the east side of the River Wylye, firm white chalk with Mgtiloides was noted in one area [8625 3817 to 8631 38221. There are two small inliers [8505 3265 and 8585 32621 of New Pit Chalk on the north side of the Mere Fault. East of the Mere Fault, firm to hard, flintless chalk was noted above the track [8715 3246 to 8722 32421 at Chapel Farm, Upton. In the road bank [8840 31161 north of East Knoyle there is brash of firm, white chalk with Mytiloides ex gr. hercynicus-subhercynicus. On the Hindon Road at East Knoyle, an old pit [8875 31091 formerly exposed 15.2 m of firm white chalk in thick beds beneath the Chalk Rock.

Lewes Nodular Chalk On the interfluve on the west side of the Wylye valley between Brixton and Monkton Deverill, chalkstone and hard nodular chalk is common on the wide outcrop of the Lewes Nodular Chalk. Glauconitised chalkstone was noted at three localities [8578 3938, 8536 3895 to 8547 3884 and 8558 38651. Nodular chalk with Gibbithyris sp. was seen at one point [8521 37751. South of Monkton Deverill, the interfluves on the west side of the tributary valley to the River Wylye are also capped by Lewes Nodular Chalk. Glauconitised chalkstone was only noted at the base of the member in two places [8501 3675 and 8512 36781. On the east side of the Wylye valley, there is a wide outcrop of Lewes Nodular Chalk extending up all the tributary valleys. In the south-west, glauconitised chalkstone was seen at one point [8552 35601. Some 350 m east, Cpemnoceramus? preserved in flint was found in brash [8585 35651. Nearby, and about 5 m higher, Micraster cortestudinarium was found (Woods, 1994b) . South-east of Monkton Deverill , glauconitised chalkstone was seen in three places [8627 3674, 8650 3668 and 8681 36631. On the opposite side of the

20 valley from this last locality is an old quarry [8695 36601. Although largely obscured, three glauconitised chalkstones, separated by 0.9 and 0.7 m of nodular chalkstone, can still be seen (Figure 3). Glauconitised chalkstone brash was also noted nearby on the north side of the valley [8712 3675, 8661 3690, 8664 3721, 8660 3718 and 8613 37581. West of this last locality, one specimen of Micraster sp. was found in brash [8626 37261. About 35 m higher, a single specimen of Cremnoceramus schloenbachi? was found loose [8662 37521 (Woods, 1994b) ; this is indicative of the cortestudinarium Zone. A well [8830 36521 at Lower Pertwood Farm proved 31.7 m of Lewes Nodular Chalk (Jukes-Browne and Hill, 1903): Thickness m Soft, but lumpy chalk with a few scattered flints 14.63 Chalk lumps with soft chalk with some layers of flint 10 * 97 Beds of solid chalk 3.66 Chalk Rock - hard rocky chalk in three beds 2.44 New Pit Chalk Solid, soft chalk with flints at two levels 12.80 ?Holgzdell Nodular Chalk Solid chalk without flints, harder below 8.84 Hard chalk, partly very hard (Melbourn Rock) 2.44 Ztg Zag Chalk Firm chalk 5.79 Marly chalk with plenty of water near bottom 24 * 99

South of Brixton Deverill, glauconitised chalkstone was seen in two areas [8639 3822 and 8665 38343. About 7 m above this last occurrence, one specimen of Cremnoceramus defformis?, suggestive of the cortestudinarium Zone, was found [8668 3831 (Woods, 1994b). East of Brixton Deverill, where there is a large outcrop on the interfluve [around 872 3891, glauconitised chalkstone is common at the base of the member [8687 3865, 8687 3899 to 8698 3936, 8755 3934, 8753 3890 to 8781 38821. Around Little Down, glauconitised chalkstone was noted at four points [8757 3843, 8800 3869, 8820 3869 and 8833 38723. On the opposite side of the valley, Micraster in flint preservation was found [8825 38821. A section [8805 39461 at Lord’ Hill Farm showed 90 cm of porcellanous chalkstone, with glauconitised hardgrounds at the top, 12 cm below and at the base of the section (Figure 3). Glauconitised chalkstone is common both west [around 8800 39471 and east [8823 39431 of the above section. Similar lithologies were seen at the eastern end of Parsonage Down [8918 3935 and 8920 39421 and near Sutton Bottom [8903 3969, 8860 3993 and 8848 39981. In the south, the Lewes Nodular Chalk has a very narrow outcrop due to

21 1.5 km south-west of Lord's Hill Farm Monkton Deverill 18805 39461 C8695 36601

large1y

obscured

3 +----

Chalkstone

Glauconitised hardground and pebbles Nodular chalk

Figure 3- Sketch of hardgrounds in the Chalk Rock the steep dips associated with the Mere structure. Hard Chalk Rock, dipping c. gonorth-north-east was noted by Mottram (Ms, BGS) on the western margin [85O6 32731 of the district. Nearby, in an old pit [8514 32661, now largely filled, Mottram noted 'Chalk Rock' on the south side. Chalk Rock also appears to have been worked in a pit [8569 32621 some 600 m farther east. Glauconitised chalkstone was found in brash close by [8575 32621. A good exposure of chalk the planus Zone was seen in a quarry [8726 32441 by the road to Chapel Farm, Upton (Jukes-Browne, Ms); the beds dip at about 25' north: Thickness m

Rather hard, white chalk with many layers of irregular flint nodules, large and small 6.10 Rather hard, white chalk with 3 continuous seams of flint 0.61 Hard, rough, yellowish rock passing into rough gritty chalk 0.46 Hard, nodular, cream-coloured chalk with a few flints, Hicrasters common 6.10 Hard limestone, with two layers of green-coated nodules, forming a ridge at the bottom of the pit 0.91

Jukes-Browne thought that planus Zone was confined to the lowest 7.5 m exposed. The basal beds (below the Chalk Rock) of the member were seen in a temporary pit [8817 31621 north of East Knoyle. There, about 1.6 m of firm to hard, white, lumpy, flintless chalk was exposed; one specimen of Spandylus spinosus was found. A good section was formerly exposed in the large quarry [8875 31101 east of East Knoyle (Jukes-Browne and Hill, 1904, p.75 and Ms) . Thickness m Lewes Nodular Chalk 1.White chalk with many flints, some nodular and some finger-shaped, also several continuous layers or seams of flint 6.10 2.RoughY nodular, sandy chalk with green grains and a few flint nodules 0.46 3.Hard, gritty nodular chalk with a layer of yellow- coated nodules at top, and others, less conspicuous below; a Micraster found 3.66 Q.Hard, yellowish limestone in one massive bed, with layers of green-coated nodules in it 1.07 New Pit Chalk and Hobgwell Nodular Chalk 5.Firm white chalk in thick beds 15.24 no lywe 11 Nodular Chalk 6.Hard cream-coloured chalk, seen for 0.61

23 Jukes-Browne and Hill (1904) regarded beds 3 and 4 as comprising the Chalk Rock. However, it seems more likely that Bed 4 is the Chalk Rock as used in this account; the nodular layer at the top of Bed 3 is probably the East Knoyle/Hope Gap Hardground (see below). The lower part of the quarry is now filled, but about 8.5 m of the upper part of the section is still visible (Woods, 1994c, loc. 7) (Figure 4). The dip in the quarry is 20° northwards. The new collecting shows that Micraster normanniae does not range above the East Knoyle Hardground in the lower part of the section. The morphological features of a Micraster from the surface of the hardground are transitional between M. normanniae and M. cortestudinarium, suggesting a correlation with the Hope Gap Hardground of Sussex, in the lower part of the cortestudinapiwn Zone (Mortimore, 1986). The fauna from the higher part of the section is dominated by Cremnoceramus, which are typical of the cortestudinariwn Zone (Mortimore, 1986). In Sussex, Cremnoceramus schloenbachi predominates above the Hope Gap Hardground (Mortimore, 1986), as it does above the East Knoyle Hardground and provides further support for the correlation. Other species of Cremnoceramus (e.g. C. ex gr. waltersdorfensis? and C. ex gr. rotundatus) above the East Knoyle Hardground, however, occur more typically below the Hope Gap Hardground in Sussex (Mortimore, 1986). A crushed fl. cortestudinarium 0.1 m above Flint 8 is consistent with a level above the Hope Gap Hardground. A specimen of M. cortestudinarium, not accurately located, occurs in hard, ironstained and glauconitised chalk, and may have come from the East Knoyle Hardground, supporting its correlation with the Hope Gap Hardground of Sussex. Up to 2.5 m of section occurs beneath the East Knoyle Hardground, but there is no unequivocal evidence of the plana Zone at this level. In Sussex, the Hope Gap Hardground is c. 5 m above the base of the cortestudinarium Zone (Mortimore, 1986). At Knoyle Corner, hard, nodular chalk and chalkstone occurs at the top of a small pit [8967 30931.

Seaford Chalk In the north-east of the district, excavations [8977 38471 for a house exposed about 2.5 m of firm, white, blocky, flinty chalk. Fragments of Platyceramus and Volviceramus involutus are common, together with a possible Micraster coranguinum (Woods, 1994b). Theco-occurrence of V. involutusandPlatyceramus indicate the lower part of the coranguinum Zone. Acme occurrences of V. invoZutus are associated with the Seaford Chalk interval containing the Seven

24 6 Nodular flint - Tabular flint z Zoophvcos W Hardground 10 // fl 4 Chalkstone

o Glauconitised nodules

!'>-*' -P .rs;9~ -- I Conspicuously nodular -'dC'chalk 9 Abundant Cremnoceramug

East Knoyle [=?Hope Gap] Hardground

1 ac rd Micraster ' normanniae

44 0 3 !x0 Chalk Rock of 'thisaccount?

Figure 4,Section in the Lewes Chalk at the East Knoyle Chalk pit E8875 31101 (based on Jukes-Browne and Hill (1904) and Woods (1994~)) Sisters Flint, about 17 m above the base of the Seaford Chalk (Mortimore, 1986). This locality lies about 35 m above the base of the Seaford Chalk (c. 18 m above the base of the coranguinum Zone). Fragments of PZatyceramus were found about 15 m lower [8977 38351, south of the above section, and near the base of the Seaford Chalk on the opposite side of the valley [8978 37751. South-south-east of Brixton Deverill, fragments of PZatyceramus were found 10 C8656 38011 and 15 m [8658 37971 above the base of the member. Further up the hill, about 23 m above the base (c.7 m above the base of the eoranguinum Zone), a specimen of VoZviceramus invozutus in flint preservation was found (Woods, 1994b). On the south side of the hill, PZatyceramus fragments were again noted [8677 3766, 8684 3766 and 8687 37651. Firm, white chalk with PZatyceramus was noted north-east [8917 36901, south-east [8880 36263 and south-west [8753 35661 of Lower Pertwood. At Pertwood, a shallow excavation [8907 35751 revealed firm white chalk with common PZatyceramus. PZatyceramus was also found in the valley bottom [8929 3545 to 8944 35431 south-east of Pertwood. PZatyceramus and a possible VoZviceramus (identification by CRB) were found as brash [8936 34401 west of Knoyle Down Farm. At West Knoyle, PZatyceramus fragments were found about 25 m above the base of the Seaford Chalk [8533 33371. The position above the base of the member of another occurrence [8575 32691 of PZatyceramus? is difficult to estimate because of probable steep dips associated with the Mere Fault. North-east of Manor Farm, West Knoyle, an old pit exposes about 2 m of firm white chalk with common PZatyceramus and VoZviceramus invozutus in two places [8619 3283 and 8619 32861. Although the bedding in the pit is apparently horizontal (Mottram, Ms, BGS), steep dips associated with the Mere Fault are likely between the pit and the fault which runs along the head- filled valley to the south. For this reason, it is difficult to work out the height of these occurrences of V. invoZutus above the base of the Seaford Chalk, but the lower is not less than 20 m and the higher not less than 25 m above the base (i.e., not less than 4 and 9 m respectively above the base of the coranguinum Zone). Some 350 m east of the pit, Micraster aff. bucaiZZei, suggestive of the coranguinum Zone, was found in brash. Farther east, PZatyceramus was found in brash [8706 32631 close to the base of the Seaford Member in an area of presumed steep dip, north of Upton, and higher in the sequence in areas of gentle dip north-west [8807 3255 to 8832 32561 and north [89OO 32551 of Sheephouse Farm, East Knoyle.

26 3. STRUCTURE The dominant structural feature of the district is the Mere Fault and associated monoclinal fold south-east of the fault. The Mere Fault, formerly known as the Great Fault (Bartlett and Scanes, 1916, p.120), was first named by Edmunds (1938, p.174). An east-west section of the Mere Fault is readily traceable over a 1 km length in the west of the district, where Chalk is faulted against Kimmeridge Clay. At West Knoyle, the fault swings slightly northwards and trend east-north-eastwards for about 500 m along a head-filled valley, beyond which it cannot be recognised as a fracture at the surface. East of the fault, there is a sinuous belt of steep dips (the overall strike is south-easterly, with dips up to 42' in a general northerly direction) traceable to the eastern margin of the district and beyond (Figure 5). To the north and north-east of this structure, the dip rapidly flattens out and subhorizontal strata, or with only a very gentle (c. 1') dip to the east or south-east, crop out over the central and northern part of the district. In the north-west of the district, a north-north-easterly trending fault along the Wylye valley has a downthrow of about 10 m east. The throw of the Mere Fault is idfficult to calculate because the amount of pre-Cretaceous erosion is not known. All that is known is that is that New Pit Chalk and Lewes Chalk on the north side of the fault are downthrown against Kimmeridge Clay which probably lies close to the Lower/Upper Kimmeridge Clay boundary. A minimum of 120 m of Cretaceous strata are cut out by the fault. Around East Knoyle, there are smaller faults associated with the monoclinal structure, in which the base of the Gault is downthrown about 15 m.

27 I I I I 86 87 88 Seaford86 Chalk -1 Lewes, Middle and Lower Chalk Gault and Upper Greensand Kimmeridge Clay and 33- Portland Beds Fault, crossmark on downthrow side Inclined strata, dip in degrees Regional dip of strata, dip in degrees

'Figure Sketch-map of the Mere Fault and associated monoclinal structure 4. DRIFT Clay-with-flints Clay-with-flints is developed on the Seaford Chalk principally in the eastern central part. The deposits vary from almost a flint gravel [particularly between 870 380 and 889 377, and around 8915 38751 with very little clay matrix, to flinty clay. The two deposits appear to merge with one another. No section has been sen through the deposit and its maximum thickness is unknown - it is probably in the range 2 to 3 m.

0 ldez? Head There are two patches of older head to the east [886 3051 and south-east [891 3001 of East Knoyle. They consist of sandy, pebbly clay with common chert clasts. A thickness of more than 2.2 m was proved in an auger hole [8884 30641.

Head Heterogeneous deposits of sandy clay, clayey sand and pebbly sandy clay and clayey sand occur principally along valley bottoms and sides. At West Knoyle, the deposits contain much chalk debris [around 860 3251. Head deposits in a slightly different situation occur along the Mere Fault where they occupy a depression up to 1.4 m deep Ce.g.8500 32611. A third situation in which head deposits occur is the sheet-like spread [865 3021 near Moor's Farm in the angle formed by the confluence of two streams. In this last situation, a thickness greater than 2.2 m of sandy clay was proved locally [8646 30291, although where it was possible to auger through the deposits, they consisted of between 1 and 2 m of sandy clay, resting on between 0.1 and 0.6 m of clayey gravel. This last occurrence may be degraded river terrace deposits. The most extensive spreads border the River Wylye and occupy the bottoms of its tributary valleys in the north-west of the district. The thickness of this spread is unknowm, but it probably rarely exceeds 2 m. At East Knoyle, 2.9 m of head deposits, consisting of 0.8 m of chalk and flint gravel, on 0.5 m of silt with some chalk fragments, on 1.4 m of silty clay with some flint gravel, on 0.2 m of silty sandy clay with some flint and chert fragments, rests on Upper Greensand. In places [e.g. 8872 3966 and 8872 39621, there is a distinct step down where a tributary valley joins a more major valley. A similar feature has been noted in the Shaftesbury area (Bristow et al., 1994, p.141). Undij'ferentiated River Terrace Deposits In the south-west of the district, patches of sandy clay with a gravelly base occupy hill tops at various heights above the present streams. The capping [856 3111 south of West Knoyle is more of a clayey gravel. Because of their gravelly and generally planar base, they are regarded as river terrace deposits, but some may be dissected remnants of sheets of older head, such as recorded in the Gillingham area to the south-west (Bristow, 1993). The maximum thickness proved in the district is greater than 1.4 m [8509 32391.

First River Terrace Deposits A spread of flinty clay, about 0.5 to 1 m above the present floodplain, borders the River Wylye in the north of the district. Its thickness is unknown.

Alluvium The principal tract of alluvium is along the River Wylye in the north-west of the district. There, the width of the alluvium varies from 40 to 150 m. No detail of the litholgy is available, but it probably consists of an upper unit of silty, organic clay, resting on a gravel. The thickness is unknown.

Lands 1 ips Several landslips involving the Gault and Kimmeridge Clay occur in the south- west of the district (Figure 2). Some [863 325, 8885 3160, 889 310, 8740 3056 and 8750 30371 are related to springs issuing from the base of the Gault, others [859 316, 861 320, 8655 3100 and 893 3001 occur on the steep sides of actively eroding streams. All those in the latter category are actively moving at the present day, as is the one [8750 30371 of the former category south of the Holloway.

Swallowholes Scattered swallowholes have been noted along some of the chalk valleys [8707 3755, 8778 3853, 8835 3650, 8865 3655, 8864 3540 and 8911 34571. 5. ECONOMIC GEOLOGY

Building stone The Ragstone at the top of the Shaftesbury Sandstone has been widely dug, presumably for building stone, in the West Knoyle-Upton area [8620 3264, 8690 3238, 8701 3231 and 8707 32281. There are pits [8722 3109 and 8758 30641 in similar situations at Milton.

Roads tone Chert from the Boyne Hollow Chert has been locally dug [8771 3179, 8772 3160 and 8771 31541, presumably for roadstone. Chert was also dug from the more easterly of the pits in the Upton area (see above). The Chalk Rock was formerly widely used as roadstone in the district, and it is probable that many of the pits [8514 3266,, 8578 3262 and 8695 36601 that were opened at this level were for roadstone. Some of the very flinty Clay-with-flints [around 880 3791 may have been worked for raodstone.

Lime and mar1 Chalk was certainly dug for lime from the large pit [8877 31111 on the east side of Hindon; there used to be a kiln in this pit. The large pit (Bull Pits) [8972 30781 at Knoyle Corner, which worked the Zig Zag Chalk, was probably opened for lime. In the north of the district, another pit [8597 39621 which was also opened in Zig Zag Chalk, may have been for lime.

Brick clay Kimmerdige Clay was worked for bricks on the west side of East Knoyle [8700 30651 6. REFERENCES Bartlett, B P, and Scanes, J. 1916. Excursion to Mere and Maiden Bradley in Wiltshire. Proceedings of the Geologists' Association, Vol. 27, 117-134.

Blake, J F, and Hudleston, W H. 1877. On the Corallian rocks of England. Quarterly Journal of the Geological Society of London, Vol. 33, 260-405.

Bristow, C R. 1993. Geology of the Gillingham - Semley district, Dorset. British Geological Survey Technical Report, WA/93/87. Bristow, C R. 1994. Geology of the Horningsham-Warminster south district, Wiltshire. British Geological Survey Technical Report, WA/94/88.

Bristow, C R, Barton, C M, Freshney, E C, Wood, C J, Evans, D J, Cox, B M, Ivimey-Cook, H 6, and Taylor, R T. 1994. Geology of the country around Shaftesbury. Memoir of the Geological Survey of Great Britain. Sheet 313 (England and Wales).

Edmunds, F H. 1938. A contribution on the physiography of the Mere district, Wiltshire, with Report of Field Meeting. Proceedings of the Geologists ' Association, Vol. 49, 174-196.

Jukes-Browne, A J, and Hill, W. 1900. The Cretaceous Rocks of Britain. 1. The Gault and Upper Greensand. Memoir of the Geological Survey of Great Br i tain.

Jukes-Browne, A J, and Hill, W. 1903. The Cretaceous Rocks of Britain. 2. The Lower and Middle Chalk of England. Memoip of the Geological Survey of Great Britain.

Jukes-Browne, A J, and Hill, W. 1904. The Cretaceous Rocks of Britain. 4. Upper Chalk of England. Memoir of the Geological Survey of Great Br i tain. Mortimore, R N. 1986. Stratigraphy of the Upper Cretaceous White Chalk of Sussex. Proceedings of the Geologists ' Association, Vol. 98, 97-139.

Wimbledon, W A. 1976. The Portland Beds (Upper Jurassic) of Wiltshire. The Wiltshire Archaeological and Natura1 History Journal, Vol. 71, 3-11.

Woods, M A. 1993d. Identification and interpretation of Chalk macrofaunas from sheets 281 (Frome), 297 (Wincanton) & 298 (Salisbury). British Geological Survey Technical Report, WH/93/291R.

Woods, M A. 1994b. Macrofaunas from the Upper Greensand and Chalk of the Wincanton (297) and Salisbury (298) sheets. British Geological Survey Technical Report, WH/94/222R.

Woods, M A. 1994~.Macrofaunas from the Upper Greensand and Chalk of the Wincanton Sheet (297): 1:lO 000 quarter sheets ST73SE, ST83NW, SW, SE, ST84SW, ST93NW & SW. British Geological Survey Technical Report, WH/94/279R.

32 Woodward, H B. 1895. The Jurassic Rocks of Britain. Vol.5. The Middle and Upper Oolitic rocks of England (Yorkshire excepted). Memoir of the Geological Survey of Great Britain.

Wright, C W, and Kennedy, W J. 1984. The Ammonoidea of the Lower Chalk: Part 1. Palaeontographical Society, 126 pp. & 40 pls.

Appendix 1. Palaeontological reports relevant to the district

Cox, B M. l994a. Recent collecting from the Kimmeridge Clay of 1:lO 000 sheets ST83SE and SW, and ST82NE. Technical Report of the British Geological Survey, No. WH/94/230R.

Cox, B M. 1994b. Further Kimmeriedge Clay material from 1: 10 000 sheets ST 83 SE and SW. British Geological Survey Technical Report No.WH/94/299R.

Woods, MA. 1993d. Identification and interpretation of Chalk macrofaunas from sheets 281 (Frome), 297 (Wincanton) & 298 (Salisbury). British Geological Survey Technical Report, WH/93/291R.

Woods, M A. 1994b. Macrofaunas from the Upper Greensand and Chalk of the Wincanton (297) and Salisbury (298) sheets. British Geological Survey Technical Report, WH/94/222R.